UCLA scientists have gone some way to explaining the mysterious “third Van Allen” belt that turned up unexpectedly last August when NASA fired up its Radiation Belt Storm Probes (RBSP) to look at the radiation region that surrounds the Earth.

Back in March, NASA said its work to calibrate the RBSPs' Relativistic Electron Proton Telescope (REPT) against the Solar, Anomalous, and Magnetospheric Particle Explorer made the unexpected discovery of an extra belt of extremely high energy electrons – moving at close to the speed of light – with no immediate explanation for why it was there.

The other exciting characteristic of the third belt was that it was long-lived: it lasted through four weeks of observations, without being scattered as expected.

Now, a team led by Yuri Shprits, a research geophysicist with the UCLA Department of Earth and Space Sciences, believes it has nailed the process that produced the third belt. In a study published in Nature Physics, they explore the near-light-speed electrons' entrapment in the Van Allen belt.

As explained in this UCLA release, there's enough energy "out there" for electrons to be accelerated to near-light-speed. However, they would be expected to be scattered by very-low-frequency plasma waves in the magnetosphere (for information on those plasma waves, Stanford's VLF Group has this handy primer).

To quote from the UCLA announcement:

Shprits and his team found that on September 1, 2012, plasma waves produced by ions that do not typically affect energetic electrons "whipped out ultra-relativistic electrons in the outer belt almost down to the inner edge of the outer belt". Only a narrow ring of ultra-relativistic electrons survived this storm. This remnant formed the third ring.

After the storm, a cold bubble of plasma around the Earth expanded to protect the particles in the narrow ring from ion waves, allowing the ring to persist.

Shprits' group also found that very low-frequency electromagnetic pulsations that were thought to be dominant in accelerating and losing radiation belt electrons did not influence the ultra-relativistic electrons.

“This study shows that completely different populations of particles exist in space that change on different timescales, are driven by different physics and show very different spatial structures,” Shprits said.

However, he adds that while the persistence of the “third belt” can now be explained, "we still need to understand fully how these electrons are accelerated".

The Van Allen belt is already known to be a dangerous place for satellites and spacecraft. The additional problem when a relativistic "third belt" exists is that it's impossible to shield against electrons moving at close to the speed of light. ®